In the field of MRI and particularly in the field of cardiac MRI, a series of images may be acquired that have different slices of the target (e.g., the patient's heart) at different times (e.g., at different times during the cardiac cycle). Typically, the temporal resolution of each “cine” scan can be approximately 30-50 milliseconds, which is generally limited only by the patient's breath-hold duration, typically 5-25 seconds. This temporal resolution is sufficient for a number of MRI applications, such as a study of a general cardiac function. This resolution may be insufficient for other applications, such as a study of extremely rapid mechanical cardiac functions, for example, valve motion, mechanical activation maps, and evaluation of mechanical dyssynchrony, which is a key indicator for early stages of systolic contraction and diastolic expansion.
To improve temporal resolution, one conventional approach can use a multi-echo steady-state free precession (“MESSFP”) technique which is known. The MESSFP technique permits temporal resolutions of approximately 5 milliseconds and acquisition of single-slice datasets in a single breath-hold scan. In this approach, each echo acquires data for a different line of frequency space (e.g., k-space), and all echoes in the train of echoes acquires data for the same cardiac phase. The MESSFP technique is phase-dependent, and requires a smooth transition from one line of k space to the next line of k space. For example, the phase referred to herein is the phase of the complex data, which can be labeled as a “complex” phase. MESSFP also requires a careful design to avoid “complex” phase-based ghosting artifacts, uses various magnetic field gradient schemes (e.g., flyback schemes, echo-shifting, etc.). These considerations limit the temporal resolution of the MESSFP technique to approximately 5 milliseconds, in a series of three (3) echoes.
Single echo SSFP (e.g., balanced or unbalanced SSFP) techniques allow for a temporal resolution of approximately 2.5 milliseconds, but suffer from certain problems. For example, single echo SSFP techniques require application of a large number of radio frequency (“RF”) pulses into the patient's body, which is undesirable. Also, for sequences that involve magnetization preparation (e.g., magnetization tagging), application of a high number of RF pulses causes an early destruction of the magnetization preparations (i.e., the tags fade faster). This potentially limits the analysis of prepared datasets (i.e., tagged datasets) to a small fraction of the cardiac cycle.
MESSFP techniques are generally used in conjunction with parallel imaging techniques, such as time-adaptive sensitivity encoding (“TSENSE”), “complex” phased array approach to ghost elimination (“PAGE”), sensitivity encoding (“SENSE”), simultaneous acquisition of spatial harmonics (“SMASH”) or Generalized Autocalibrating Partially Parallel Acquisitions (“GRAPPA”), to speed up data acquisition. The reconstruction based on this combination can be a lengthy process because each echo in the train of echoes (or echo-train) contributes to the same image, or cardiac phase, and it may not be easy to parallelize the reconstruction of the data acquired using this combination.
Magnetic resonance “complex” phase velocity mapping (PVM) using a “complex” phase contrast approach is a procedure that has been used clinically to measure blood flow (e.g., see G. P. Chatzimavroudis et al., “Evaluation of the precision of magnetic resonance phase velocity mapping for blood flow measurements,” J Cardiovasc Magn. Reson. 2001; 3(1):11-9). However, conventional “complex” phase contrast (PC) magnetic resonance imaging may have a lower temporal resolution than corresponding magnitude imaging, due to the preference to acquire two differentially flow-encoded images for every PC image frame, in order to subtract out or remove non-motion-related “complex” phase changes.
Spiral “complex” phase contrast pulse sequences have been used (e.g., see K. S. Nayak et al., “Real-time color flow MRI,” Magn Reson Med 2000; 43(2):251-8) to develop real-time color flow MRI. Since this approach is real-time, no breath-holding or gating is needed. In this approach, a sliding window reconstruction can be used to acquire the data. However, this approach can only yield temporal resolution of up to 60 msec (e.g., when used with single-shot spirals), with a corresponding spatial resolution of 4 mm.
The used of balanced steady-state free precession (b-SSFP) has been demonstrated (e.g. see M. Markl et al., “Balanced phase-contrast steady-state free precession (PC-SSFP): a novel technique for velocity encoding by gradient inversion,” Magn Reson Med 2003; 49(5):945-52) to perform “complex” phase contrast imaging (PC-SSFP). b-SSFP sequences are attractive since they exhibit intrinsically higher signal-to-noise ratios (SNRs) than conventional imaging sequences. The publication by Markl, et al. demonstrates an approach to encode for flow without introducing any additional velocity encoding gradients, so as to keep the repetition time (TR) as short as in typical SSFP sequences. Instead, sensitivity is established to through-plane velocities by inverting (i.e., negating) all gradients along the slice-select direction. It was possible to adjust the velocity encoding sensitivity (Venc) by altering the first moments of the slice-select gradients. In order to avoid disturbing the SSFP steady state, they acquired different flow echoes in sequentially executed scans, each over several cardiac cycles, using separate steady-state preparation periods. Using this approach, it was possible to show that PC-SSFP exhibited a higher intrinsic SNR and consequently lower “complex” phase noise in measured velocities compared to conventional “complex” phase contrast (PC) scans. It was also demonstrated that PC-SSFP is less reliant on in-flow-dependent signal enhancement, and hence yields more uniform SNR and better depiction of vessel geometry throughout the cardiac cycle in structures with slow and/or pulsatile flow. Their acquisition, however, had a temporal resolution of only 58 ms.
The highest temporal resolution “complex” phase contrast MRI has been demonstrated (e.g., see R. B. Thompson et al., “High temporal resolution phase contrast MRI with multiecho acquisitions,” Magn Reson Med 2002; 47(3):499-512) by using a multiecho acquisition. In this publication, an improvement by a factor of 2 in the temporal resolution was achieved by acquiring the differentially flow-encoded images in separate breath-holds rather than interleaved within a single breath-hold. They also utilized the multiecho readout to acquire more views per unit time than is possible with the conventional PCMRI sequence. These changes allowed them to achieve a total improvement in temporal resolution by ˜5 times over conventional PC imaging. They were able to achieve a temporal resolution of 11.2 ms and an in-plane spatial resolution of 2 mm×2 mm. Yet higher temporal resolution than that achieved to date is desirable which will allow the acquisition of very high resolution velocity data. This can be used, for example, for the evaluation of valvular function and flow mechanics. The present invention provides for finer detail of jets that may arise from regurgitation and/or stenosis. The present invention permits the highest temporal resolution velocity measurement with MRI than that which is known to have ever been recorded before.
Single-shot magnetic resonance imaging, in which the data corresponding to the entire image is acquired in a single instance, can generally be used to acquire snapshot images of physiologic processes. While echo-planar imaging (EPI) is one of the renowned examples of fast snapshot imaging, this technique may suffer from various artifacts, such as susceptibility, and T2 or T2* signal attenuation effects, which reduce its practical usefulness to select regions of the body, and to low spatial resolutions. One potential region where there is a preference for fast snapshot imaging is studying lung dynamics using hyperpolarized helium (3He) (or similar polarized or hyperpolarized gas) MRI. Due to the short T2* of 3He (approximately 12 ms), EPI is not a practical solution for clinically useful spatial resolutions (3 mm or less). While techniques such as spiral imaging have been developed for fast imaging, such techniques have to utilize sliding window approaches to achieve “high” temporal resolution. Beside the situation that spiral imaging may be nontrivial to reconstruct, the sliding window approach can lead to an averaging of the data over multiple time points, making it difficult to determine the true temporal resolution of the data. Segmented and multi-echo gradient recalled approaches have been developed to overcome some of the artifacts associated with conventional EPI techniques. These approaches acquire only a short echo train corresponding to a portion of the data representing the image after every RF pulse. This approach can enable data acquisition within T2* limits, may minimize the susceptibility artifact, and can allow higher spatial resolution to be achieved. However, this segmented approach can make it difficult to acquire data with the type of temporal resolution that is achieved with EPI.
High temporal resolution dynamical imaging of lungs using hyperpolarized gases such as 3He can be used for diagnosis of many pulmonary diseases. Different techniques have been developed to achieve this. For example, fast EPI has been developed to achieve temporal resolution of 40 ms (see, e.g., Saam et al., MRM. 1999; 42:507-514)) for matrix size 32×64. A projection reconstruction method (see, e.g., Holmes et al., Proc. ISMRM, Miami, 2005) provides a temporal resolution of 366 ms for matrix size 256×256. Interleaved spiral imaging can provide a temporal resolution of 3.4 ms for matrix size 128×128 using 24 interleaves and a sliding window technique for reconstruction (see e.g. Salerno et al., MRM 2001 46:667-677). However, spiral imaging can be cumbersome to implement and the sliding window reconstruction leads to time averaging of the signal. One of the major problems encountered in making accurate maps of T1 is the long time generally needed. For a good accuracy over a wide range of T1 values, multiple points of the T1 recovery curve should be sampled. If a conventional two-dimensional (2D) inversion-recovery spin-echo sequence is used, data acqisition for each slice can take a significant time (e.g., several hours). Several schemes have been developed for the rapid imaging of T1 in 2D in which multiple points on the recovery curve can be sampled. These techniques can include methods based on Look-Locker, snapshot-fast low-angle shot (snapshot—FLASH), inversion prepared echo planar imaging, and stimulated echo imaging.